Apart from others mentioned, Kingfisher seems like one of the happiest members to me. Content in himself and his situation. Quinlan too, quiet contentment. That to me seems like true happiness, having a measure of that feeling in you.
I think Halla is pretty happy. But I don't see his upbeat nature as a sign of that happiness. To me, happiness is a kind of contentment coupled with a strength in self to deal with what life throws at you.
The happiest person I ever knew was my granddad. Almost never said a word, just went about his way.